In a mobile communication system, for example, when a mobile station transitions from an idle state to a call request procedure, since a dedicated channel has yet to be set, the mobile station uses a random access channel (which corresponds to a physical random access channel of the physical layer and is a common transport channel) and transmits a preamble as a reference signal. The preamble includes a sequence having favorable autocorrelation properties. The preamble is used for uplink signal synchronization between the mobile station and a base station.
A method of uplink signal synchronization between a mobile station and a base station using a preamble will be described with reference to FIG. 1. FIG. 1 is a timing chart depicting preamble transmission and reception timing between a mobile station and a base station. As depicted in FIG. 1, when the uplink signals between the mobile station and the base station are not synchronized, the mobile station receives signals transmitted from the base station at sub-frames (Sub-frame #1, #2, . . . ). The mobile station transmits a preamble to the base station at the time when reception of downlink sub-frames (in FIG. 1, Sub-frame #3) is completed. Assuming the communication depicted in FIG. 1 is performed, at the base station, deviation (delay) of the time at which the preamble is received with respect to the time at which transmission of Sub-frame #3 ends, includes the downlink delay and the uplink delay. The base station, by correlating a sequence included in the received preamble and known sequences, calculates the delay. The calculated delay is fed back to the mobile station and used for uplink signal synchronization.
As the distance between the base station and the mobile station increases, the delay also increases. Thus, under next generation mobile communication standards (Evolved Universal Terrestrial Radio Access (E-UTRA) also called Long Term Evolution (LTE)), as depicted in FIG. 2, multiple preamble formats (Preamble formats: 0, 1, 2, 3) are prepared according to the size of the operating cell. A cyclic prefix (CP) is extracted from the tail of a single, fixed length sequence and can be considered as a portion of the sequence. Under LTE, 64 types of Zadoff-Chu sequences are used for the preamble. A Zadoff-Chu sequence is a constant amplitude zero auto-correlation (CAZAC) sequence having favorable autocorrelation properties.
The mobile station arbitrarily selects any one among the 64 types of Zadoff-Chu sequences, generates a preamble, and transmits the preamble to the base station. The base station correlates the received preamble sequence and the known 64 types of sequences (replicas) and thereby detects the type of sequence used. The base station further autocorrelates the detected sequence and thereby calculates the delay (deviation of preamble reception time). In other words, a sequence received within an interval (hereinafter, sequence detection interval) preliminary set based on a given reference time (e.g., the time at which transmission of a given sub-frame ends) is cyclically shifted according to the delay of preamble reception. Consequently, the base station, by calculating the shift amount (the position where autocorrelation peaks) of the sequence received within the sequence detection interval, calculates the delay and by feeding back the calculated delay to the mobile station, enables the mobile station to synchronize uplink signals.
Further, conventionally in connection with preambles, a method of detecting preamble code in an environment where carrier frequencies between the base station and the mobile station are offset is known.
For examples of conventional technologies, refer to Japanese Laid-Open Patent Publication No. 2008-236744 and 3GPP TS 36.211 V8.7.0: 3rd Generation Partnership Project, Technical Specification Group Radio Access Network, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation” Chapter 5.7 “Physical random access channel”.
Under LTE communication standards, a preamble format including 2 fixed length sequences is prescribed (e.g., Preamble format: 3 depicted in FIG. 2). When such a preamble format is used, since the delay may be errantly calculated, a problem arises in that the cell radius (i.e., the distance of the base station and the mobile station) enabling coverage is limited. This problem will be described with reference to FIGS. 3 and 4.
Two mobile stations (MS) #1, MS#2 at respectively different distances from a base station are assumed. In this example, the mobile station MS#2 is assumed to be located farther away from the base station than the mobile station MS#1. FIGS. 3 and 4 are timing charts depicting the timing at which preambles transmitted from the mobile stations MS#1, MS#2 are received at the base station. In FIGS. 3 and 4, time t0 is the time at which transmission of a given sub-frame from the base station to each of the mobile stations ends and is the reference time used to calculate the delay of the preambles. FIG. 4 depicts an example when the mobile station MS#2 is located farther from the base station than in the example depicted in FIG. 3.
In FIGS. 3 and 4, (a) the timing of sub-frame transmission by the base station, from the reference time; (b) the timing of preamble reception at base station, assuming no delay (delay=0); (c) the timing at which the preamble from the mobile station MS#1 is received at the base station; and (d) the timing at which the preamble from the mobile station MS#2 is received at the base station are depicted. The preambles are, for example, signals of Preamble format: 3 under LTE communication standards. In this example, as depicted by (b), the sequence detection interval is set as an interval (time t1 to time t2) that corresponds to the second sequence in a preamble assumed to not be subject to delay.
As depicted by (c) and (d) in FIG. 3, provided that the delays from the mobile stations are within a range that is not that long, differences in the delay are equivalent to differences in the sequence shift amount in the sequence detection interval. In this case, since the sequence shift amounts (the position where autocorrelation peaks) at (c) and (d) in FIG. 3 differ, each delay can be calculated without error.
Next, as depicted in FIG. 4, the delay of the mobile station MS#2 is greater than that in the example depicted in FIG. 3. Consequently, the timing at which the second sequence in the preamble from the mobile station MS#1 is received at the base station and the timing at which the first sequence in the preamble from the mobile station MS#2 is received at the base station, substantially coincide. As a result, in the sequence detection interval, since the sequence shift amounts are recognized as being equivalent by the base station, upon calculation at the base station, the same delay is calculated for the mobile station MS#1 and the mobile station MS#2. Therefore, according to the method of calculating sequence shift amounts in the sequence detection interval, error-free calculation of the delay of a preamble from a mobile station is under the condition that the mobile station is not located such a far distance away from the base station, that a delay corresponding to the reception time of a single sequence arises. To meet this condition, the distance of the mobile station from the base station when the period of time corresponding to the fixed sequence length is 800 μs, such as under LTE, is limited to within 120 km (800 μs/(6.7 μs/km)=120 km) with consideration of uplink and downlink delays.